Leachate reinjection in landfills enhances biodegradation processes and may decrease post-closure landfill monitoring. One of the main challenges with respect to the bioreactor methodology, concerns the design of the leachate reinjection system geometry and of reinjection procedures. Although 2-D electrical resistivity tomography (ERT) has been used in the past to monitor moisture in a landfill, leachate reinjection bulbs are three-dimensional and therefore 3D electrical resistivity tomography should provide more reliable information on moisture variations in the three dimensions. This paper shows how innovative fast geophysical instrumentation, with a 10-minute period monitoring in 3 dimensions during and after leachate reinjection, can provide 3-D images of resistivity variations over time. The technique can be useful for assessing the efficiency of leachate injection drain design and also to verify where biodegradation processes are occurring.